In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, surface capacitive, projected capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with an optically transparent conductive layer are separated by spacers and face each other so as to function as a touchsensor formed of an optically transparent electrode. A current is applied to the optically transparent conductive material and the voltage of the glass plate with an optically transparent conductive layer is measured. In contrast, a capacitive touchscreen has a basic configuration in which a touchsensor formed of an optically transparent electrode is an optically transparent conductive material having an optically transparent conductive layer provided on a base material and there are no movable parts. Capacitive touchscreens are used in various applications due to their high durability and high light transmission rate. Further, a touchscreen utilizing projected capacitive technology allows simultaneous multipoint detection, and therefore is widely used for smartphones, tablet PCs, etc.
As an optically transparent conductive material used for touchscreens, those having an optically transparent conductive layer made of an ITO (indium tin oxide) film formed on a base material have conventionally been used. However, there has been a problem of decrease in light transmission rate due to high refractive index and high surface light reflectivity of ITO conductive films. In addition, due to low flexibility, the ITO conductive film is prone to crack when bent, resulting in increased electric resistance of the optically transparent conductive material.
Known as an optically transparent conductive material having, on an optically transparent support, an optically transparent conductive layer different from an ITO conductive film is an optically transparent conductive material having a mesh pattern of metal thin lines with appropriately adjusted line width, pitch, pattern shape, etc., for example. This technology provides an optically transparent conductive material which maintains a high light transmittance and which has a high electrical conductivity. Regarding the shape of the mesh pattern of metal thin lines (hereinafter also written as metal pattern), it is known that a repetition unit of any shape can be used. For example, in Patent Literature 1, a triangle, such as an equilateral triangle, an isosceles triangle, and a right triangle; a quadrangle, such as a square, a rectangle, a rhombus, a parallelogram, and a trapezoid; a (regular) n-sided polygon, such as a (regular) hexagon, a (regular) octagon, a (regular) dodecagon, and a (regular) icosagon; a circle; an ellipse; and a star, and a combinational pattern of two or more thereof are disclosed.
As a method for producing the above-mentioned optically transparent conductive material having a metal mesh pattern, a semi-additive method for forming a metal pattern, the method comprising forming a thin catalyst layer on a support, forming a resist pattern on the catalyst layer, forming a laminated metal layer in an opening of the resist by plating, and finally removing the resist layer and the base metal protected by the resist layer, is disclosed in, for example, Patent Literature 2, 3, etc.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed using a silver halide photosensitive material as a precursor to a conductive material has come to be known. For example, disclosed in Patent Literature 4, 5, 6, etc. is a technique for forming a metal (silver) pattern by subjecting a silver halide photosensitive material (a conductive material precursor) having, on a support, a physical development nuclei layer and a silver halide emulsion layer in this order to a reaction with a soluble silver halide forming agent and a reducing agent in an alkaline fluid. The patterning by this method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods. An additional advantage is that a layer having a metal pattern obtained by this method has a higher flexibility, i.e. a longer flexing life as compared with an ITO conductive layer.
When an optically transparent conductive material having, on an optically transparent support, such a metal pattern as described above is placed over a liquid crystal display, the cycle of the metal pattern and the cycle of the liquid crystal display element interfere with each other, causing a problem of moire. In recent years, liquid crystal displays of different resolutions have been used, which further complicates the above-mentioned problem.
As a solution to this problem, suggested in Patent Literature 7, 8, 9, 10, etc., for example, is a method in which the interference is suppressed by using, as the metal thin line pattern, a traditional random diagram described in, for example, “Mathematical Models of Territories, Introduction to Mathematical Engineering through Voronoi diagrams” (Non Patent Literature 1).